In recent years, LED lamps have been widely used as a highly energy-efficient replacement for incandescent light bulbs. One type of white LED light source is a combination of gallium nitride (GaN) blue LEDs and YAG yellow phosphor. Such an LED light source produces blue light from the blue LED, which excites the yellow phosphor particles, in turn producing yellow light. The combination of the blue light and the yellow light yields white light.
Typically, objects viewed under a light source should preferably appear to have natural coloring. In other words, a high color rendering index is preferable. Various conventional forms of technology have been proposed for enhancing the color rendering index of an LED light source. For example, Patent Literature 1 proposes including neodymium oxide particles in a filter element attached to the LED light source as a means of improving the general color rendering index Ra. Neodymium oxide is known as a material for filters that selectively absorbs light in a wavelength band around 580 nm (for example, see Patent Literature 2). Japanese Industrial Standard (JIS) Z8726 defines methods for evaluating the color rendering index under a light source by using color rendering indices that quantitatively assess the fidelity of color reproduction under a lamp in comparison to a reference light source.